Electronic weighing apparatus utilizing surface acoustic waves

ABSTRACT

A weighing apparatus includes a base supporting a cantilevered elastic member bearing a load platform. The interior of the elastic member is hollowed and is provided with first and second piezoelectric transducers mounted on respective opposed posts. The transducers are arranged substantially parallel to each other with a small gap between them and are coupled to an amplifier to form a “delay line” and a positive feedback loop, i.e. a natural oscillator. According to various aspects of the invention, one or both substrates is provided with anti-reflection structure; the transducers are sealed from moisture; an hermetically sealed temperature transducer is provided to compensate for the effects of temperature gradients; a thermal transducer channel is provided on the same substrate to measure the effects of temperature gradients and thereby compensate for temperature effects; a pair of differential transducers is arranged to measure the effects of temperature changes in the same acoustic channel in which displacement measurements are made; and a phase shift is applied to the output of the amplifier(s) to improve gain.

This application is a continuation-in-part of co-owned application Ser.No. 08/729,752 filed Oct. 7, 1996, now U.S. Pat. No. 5,910,647, whichwas a continuation 08/489,365 filed Jun. 12, 1995, now U.S. Pat. No.5,663,531, the complete disclosures of which are hereby incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to electronic weighing devices. More particularly,the invention relates to an electronic weighing device which employssurface acoustic waves to measure weight.

2. State of the Art

Precision electronic weighing devices are widely known in the art andthere are many different technologies utilized in these electronicweighing devices. Laboratory scales or “balances” typically have acapacity of about 1,200 grams and a resolution of about 0.1 gram,although scales with the same resolution and a range of 12,000 grams areavailable. The accuracy of these scales is achieved through the use of atechnology known as magnetic force restoration. Generally, magneticforce restoration involves the use of an electromagnet to oppose theweight on the scale platform. The greater the weight on the platform,the greater the electrical current needed to maintain the weight. Whilethese scales are very accurate (up to one part in 120,000), they areexpensive and very sensitive to ambient temperature. In addition, theirrange is relatively limited.

Most all other electronic weighing devices use load cell technology. Inload cell scales, the applied weight compresses a column which hasstrain gauges bonded to its surface. The strain gauge is a fine wirewhich undergoes a change in electrical resistance when it is eitherstretched or compressed. A measurement of this change in resistanceyields a measure of the applied weight. Load cell scales are used innon-critical weighing operations and usually have a resolution of aboutone part in 3,000. The maximum resolution available in a load cell scaleis about one part in 10,000 which is insufficient for many criticalweighing operations. However, load cell scales can have a capacity ofseveral thousand pounds.

While there have been many improvements in electronic weighingapparatus, there remains a current need for electronic weighingapparatus which have enhanced accuracy, expanded range, and low cost.

Co-owned application Ser. No. 08/489,365, previously incorporated byreference herein, discloses an electronic weighing apparatus having abase which supports a cantilevered elastic member upon which a loadplatform is mounted. The free end of the elastic member is provided witha first piezoelectric transducer and a second piezoelectric transduceris supported by the base. Each transducer includes a substantiallyrectangular piezoelectric substrate and a pair of electrodes imprintedon the substrate at one end thereof, with one pair of electrodes actingas a transmitter and the other pair of electrodes acting as a receiver.The transducers are arranged with their substrates substantiallyparallel to each other with a small gap between them and with theirrespective electrodes in relatively opposite positions. The receiverelectrodes of the second transducer are coupled to the input of anamplifier and the output of the amplifier is coupled to the transmitterelectrodes of the first transducer. The transducers form a “delay line”and the resulting circuit of the delay line and the amplifier is apositive feedback loop, i.e. a natural oscillator. More particularly,the output of the amplifier causes the first transducer to emit asurface acoustic wave (“SAW”) which propagates along the surface of thefirst transducer substrate away from its electrodes. The propagatingwaves in the first transducer induce an oscillating electric field inthe substrate which in turn induces similar SAW waves on the surface ofthe second transducer substrate which propagate in the same directionalong the surface of the second transducer substrate toward theelectrodes of the second transducer. The induced waves in the secondtransducer cause it to produce an alternating voltage which is suppliedby the electrodes of the second transducer to the amplifier input. Thecircuit acts as a natural oscillator, with the output of the amplifierhaving a particular frequency which depends on the physicalcharacteristics of the transducers and their distance from each other,as well as the distance between the respective electrodes of thetransducers.

When a load is applied to the load platform, the free end of thecantilevered elastic member moves and causes the first transducer tomove relative to the second transducer. The movement of the firsttransducer relative to the second transducer causes a change in thefrequency at the output of the amplifier. The movement of the elasticmember is proportional to the weight of the applied load and thefrequency and/or change in frequency at the output of the amplifier canbe calibrated to the displacement of the elastic member. The frequencyresponse of the delay line is represented by a series of lobes. Eachmode of oscillation is defined as a frequency where the sum of thephases in the oscillator is an integer multiple of 2π. Thus, as thefrequency of the oscillator changes, the modes of oscillation movethrough the frequency response curve and are separated from each otherby a phase shift of 2π. The mode at which the oscillator will oscillateis the one having the least loss. The transducers are arranged such thattheir displacement over the weight range of the weighing apparatuscauses the oscillator to oscillate in more than one mode. Therefore, thechange in frequency of the oscillator as plotted against displacement ofthe transducers is a periodic function. There are several different waysof determining the cycle of the periodic function so that the exactdisplacement of the elastic member may be determined. In addition, inorder to minimize the possibility that the oscillator will oscillate intwo modes at the same time, the frequency response of the delay line isarranged so that no more than two modes coexist in the main lobe of thefrequency response curve. This is achieved by the topology of theelectrodes as well as the distance between the transmitting electrodeand the receiving electrode. The gain of the amplifier is also chosen tobe at least the absolute value of the greatest loss expected to beencountered at an oscillating frequency within the main lobe but notgreat enough to allow oscillation in two modes simultaneously.

According to a disclosed preferred embodiment, the surface acoustic wavehas a wavelength of approximately 200 microns at 20 MHz. The gap betweenthe substrates of the first and second transducers is as small aspossible and preferably is less than 0.1 wavelength, i.e. 10-20 microns.The amplifier preferably has a gain of at least approximately 17 dB inorder to guarantee natural oscillation, and preferably not more thanapproximately 30 dB so that the oscillator oscillates in only one modeat a time. The preferred manner of determining the cycle of the periodicoutput of the amplifier is to provide a second pair of transducersadjacent to the first pair and coupled to each other in the same type ofdelay line feedback loop. The second pair of transducers utilize a SAWwith a different wavelength than the first pair of transducers, e.g.approximately 220 microns at 18 MHz. The output of the second amplifieris, therefore, a periodic function with a different frequency than theperiodic function which is the output of the first amplifier. Bycombining the outputs of both amplifiers, a unique value is provided foreach position of the elastic member.

Typically, the elastic member is chosen so that it will bend up to 150microns under maximum load. Given the wavelength of the SAW, thisresults in about two to three modes of oscillation in the output of thefirst amplifier.

The provided apparatus can theoretically achieve an accuracy on theorder of one part in one hundred thousand, e.g. one gram per hundredkilograms. In practice, however, a resolution on the order of one partin fifty thousand is readily achieved. It has been observed by theinventors herein that several factors have varying influence on theaccuracy of the SAW system. These factors include reflected waves,temperature changes, and the frequency of the oscillator. Generally,reflected waves result in non-linearity of measurements, and temperaturehas an effect of about 70 ppm per degree C.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an electronicweighing apparatus which is accurate.

It is also an object of the invention to provide an electronic weighingapparatus which uses surface acoustic waves and is accurate over a broadrange of weights.

It is another object of the invention to provide an electronic weighingapparatus which is compact and easy to construct.

It is a further object of the invention to provide an electronicweighing apparatus which is inexpensive to manufacture.

It is another object of the invention to provide an electronic weighingapparatus which utilizes surface acoustic waves and which is providedwith means for reducing reflected waves.

It is still another object of the invention to provide an electronicweighing apparatus which maintains accuracy despite temperaturegradients within the system.

It is yet another object of the invention to provide an electronicweighing apparatus which utilizes surface acoustic waves at a relativelyhigh frequency.

In accord with these objects which will be discussed in detail below,the improved weighing apparatus of the present invention includes a basewhich supports a cantilevered elastic member upon which a load platformis mounted. The interior of the elastic member is hollowed and isprovided with first and second piezoelectric transducers which aremounted on respective opposed posts. Each transducer includes asubstantially rectangular piezoelectric substrate and a pair ofelectrodes imprinted on the substrate at one end thereof, with one pairof electrodes acting as a transmitter and the other pair of electrodesacting as a receiver. The transducers are arranged with their substratessubstantially parallel to each other with a small gap between them andwith their respective electrodes in relatively opposite positions. Thereceiver electrodes of the second transducer are coupled to the input ofan amplifier and the output of the amplifier is coupled to thetransmitter electrodes of the first transducer. The transducers form a“delay line” and the resulting circuit of the delay line and theamplifier is a positive feedback loop, i.e. a natural oscillator. Moreparticularly, the output of the amplifier causes the first transducer toemit a surface acoustic wave (“SAW”) which propagates along the surfaceof the first transducer substrate away from its electrodes. Thepropagating waves in the first transducer induce an oscillating electricfield in the substrate which in turn induces similar SAW waves on thesurface of the second transducer substrate which propagate in the samedirection along the surface of the second transducer substrate towardthe electrodes of the second transducer. The induced waves in the secondtransducer cause it to produce an alternating voltage which is suppliedby the electrodes of the second transducer to the amplifier input. Thecircuit acts as a natural oscillator, with the output of the amplifierhaving a particular frequency which depends on the physicalcharacteristics of the transducers and their distance from each other,as well as the distance between the respective electrodes of thetransducers.

According to the invention, when a load is applied to the load platform,the cantilevered elastic member bends and causes the first transducer tomove relative to the second transducer. The movement of the firsttransducer relative to the second transducer causes a change in thefrequency at the output of the amplifier. The bending movement of theelastic member is proportional to the weight of the applied load and thefrequency and/or change in frequency at the output of the amplifier canbe calibrated to the displacement of the elastic member.

According to one aspect of the invention, one or both substrates areprovided with anti-reflection structure which may be an angled cut, arounded end, or a surface damper.

According to a second aspect of the invention, the transducers arearranged on overlapping substrates which allows more room for a dampingmaterial to further reduce reflection and allows more room foradditional transducers.

According to a third aspect of the invention, the transducers arecoupled to a thermal sink to reduce the effects of thermal gradientsacross the transducers.

According to a fourth aspect of the invention, two pairs of transducersare provided and arranged to move in opposite directions which doublesthe readability of measurements and also compensates for the effects oftemperature gradients.

According to a fifth aspect of the invention, a thermal transducerchannel is provided on the same substrate to measure the effects oftemperature and thereby compensate for temperature effects.

According to a sixth aspect of the invention, a pair of differentialtransducers is arranged to measure the effects of temperature changes inthe same acoustic channel in which displacement measurements are made.

According to a seventh aspect of the invention, a phase shift(preferably 180°) is introduced in the oscillator of the delay line,when required, in order for the oscillator to oscillate in the mostoptimal section of the frequency response curve (near the center) wheretemperature effects are minimized.

According to an eighth aspect of the invention, two surface dampers areprovided for each transducer. This is accomplished in one of two ways.According to one way, a surface mount damper is formed from a thin mylarfilm. According to the other way, a multistrip coupler is formed by analuminized pattern of lines behind the transducer and a surface damperis provided behind the multistrip coupler.

According to a ninth aspect of the invention, long term stability isenhanced by sealing the transducer, preferably hermetically, and/or byproviding a second hermetically sealed temperature transducer and byusing the output of the sealed transducer to correct for the effects oftemperature and humidity.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation view of an exemplary embodiment ofthe invention;

FIG. 1a is an enlarged schematic plan view of a first transducer;

FIG. 1b is an enlarged schematic plan view of a second transducer;

FIG. 2 is an enlarged schematic plan view of a transducer having a firstanti-reflection structure according to the invention;

FIG. 3 is an enlarged schematic side elevation view of a transducerhaving a second anti-reflection structure according to the invention;

FIG. 4 is an enlarged schematic side elevation view of a transducerhaving a third anti-reflection structure according to the invention;

FIG. 5 is an enlarged schematic side elevation view of an overlappingtransducer system having anti-reflection structure according to theinvention;

FIG. 6 is an enlarged schematic side elevation view of an overlappingdifferential transducer system according to the invention, incorporatinganti-reflection structure and thermal sinks;

FIG. 7 is an enlarged schematic plan view of one pair of transducers ofthe system of FIG. 6;

FIG. 8 is an enlarged schematic plan view of the other pair oftransducers of the system of FIG. 6;

FIG. 9 is an enlarged schematic side elevation view of a differentialtransducer system according to the invention, incorporating a splitsecond channel;

FIG. 10 is an enlarged schematic plan view of one pair of transducers ofthe system of FIG. 9;

FIG. 11 is an enlarged schematic plan view of the other pair oftransducers of the system of FIG. 9;

FIG. 12 is an enlarged schematic plan view of the first part of adifferential transducer system having two pair of separate thermaltransducer channels;

FIG. 13 is an enlarged schematic plan view of the second part of adifferential transducer system having two pair of separate thermaltransducer channels;

FIG. 14 is a schematic transparent view of the transducers of FIGS. 12and 13 in an operative alignment;

FIG. 15 is an enlarged schematic side elevation view of a transducersystem having a thermal transducer located in the same acoustic channelas the displacement transducers and an example of a circuit for thesame;

FIG. 16 is an enlarged schematic plan view of the first part of thetransducer system of FIG. 15;

FIG. 17 is an enlarged schematic plan view of the second part of thetransducer system of FIG. 15;

FIG. 18 is an enlarged schematic side elevation view of a differentialtransducer system having two transducers sharing the same acousticchannel;

FIG. 19 is an enlarged schematic plan view of the first part of thetransducer system of FIG. 18;

FIG. 20 is an enlarged schematic plan view of the second part of thetransducer system of FIG. 18;

FIGS. 21-26 are a graphs of a portion of a frequency response curve fora delay line according to the invention showing modes of oscillation andphase shifting according to the invention;

FIG. 27 is a schematic diagram of a positive feedback loop with phaseshifting according to the inventions

FIG. 28 is an enlarged schematic plan view of a transducer according tothe invention illustrating the propagation of SAW waves;

FIG. 29 is a schematic transparent view of two transducers of the typeshown in FIG. 28 in operative alignment;

FIG. 30 is an enlarged schematic side elevation view of the transducersystem of FIG. 29;

FIG. 31 is a view similar to FIG. 28 showing one embodiment of atransducer with two surface dampers;

FIG. 32 is a view similar to FIG. 28 showing another embodiment of atransducer with two surface dampers;

FIG. 33 is a view similar to FIG. 29 showing two of the transducers ofFIG. 32 in operative alignment;

FIG. 34 is a view similar to FIG. 1 illustrating one way of sealing thetransducers; and

FIG. 35 is a view similar to FIG. 1 illustrating another way of sealingthe transducers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1, 1 a, and 1 b, an electronic weighing apparatus10 according to the invention includes a base 12 which supports acantilevered elastic member 14 having a cut-out 15, and upon which aload platform 16 is mounted. The cut-out 15 is provided with two opposedposts 17, 19 upon which are respectively mounted a first piezoelectrictransducer 20 and a second piezoelectric transducer 22. The posts 17, 19serve to locate the transducers 20, 22 at the center of the elasticmember 14 and to mechanically couple the transducers to opposite ends ofthe elastic member 14.

The first transducer 20 includes a substantially rectangularpiezoelectric substrate 20 a and a pair of electrodes 20 b imprinted onthe substrate at the upper end thereof. The second transducer 22includes a substantially rectangular piezoelectric substrate 22 a and apair of electrodes 22 b imprinted on the substrate at the lower endthereof. The substrates are preferably made of Lithium Niobate. Thetransducers are arranged with their substrates substantially parallel toeach other with a small gap “g” between them. The electrodes 22 b of thesecond transducer 22 are coupled to the input of an amplifier (notshown) powered by a power source (not shown) and the output of theamplifier is coupled to the electrodes 20 b of the first transducer 20.The circuit arrangement is the same as shown in the parent applicationSer. No. 08/489,365, previously incorporated herein by reference. Theresulting circuit is a positive feedback loop natural oscillator, a“delay line”. The output of the amplifier generates an alternatingvoltage in the electrodes 20 b of the first transducer 20 whichgenerates a surface acoustic wave (“SAW”) 26 which propagates along thesurface of the first transducer substrate 20 a away from its electrodes20 b. Since the substrate 20 a of the first transducer 20 is relativelyclose to the substrate 22 a of the second transducer 22, an oscillatingelectric field which is induced as a result of the SAW waves 26 in thepiezoelectric substrate 20 a is able to in turn induce similar SAW waves28 on the surface of the second transducer substrate 22 a whichpropagate in the same direction along the surface of the secondtransducer substrate toward the electrodes 22 b of the second transducer22. The induced waves 28 in the second transducer 22 cause the electrode22 b of the second transducer 22 to produce an alternating voltage whichis provided to the input of the amplifier. As long as the gain of theamplifier 24 is larger than the loss of the system, the circuit acts asa natural oscillator with the output of the amplifier having aparticular frequency which depends on the physical characteristics ofthe transducers and their distance from each other, as well as thedistance between the respective electrodes of the transducers. Inparticular, the frequency of the oscillator is directly related to thetime it takes for the SAW to propagate from the electrodes 20 b to theelectrodes 22 b.

According to presently preferred embodiments of the invention, describedin more detail below, the SAW 26 has a wavelength of approximately100-200 microns at 20-50 MHz. In order to limit loss in the system, thegap “g” between the substrates of the first and second transducers is assmall as possible and preferably no more than 0.1 wavelength. In onepreferred embodiment described below, the gap is 5-10 microns. With sucha gap, an oscillating system can typically be generated if the amplifier24 has a gain of at least approximately 17 dB. It will be appreciatedthat when a load (not shown) is applied to the load platform 16, thefree end of the cantilevered elastic member 14 moves down and causes thesecond transducer 22 to move relative to the first transducer 20. Inparticular, it causes the electrodes 22 b of the second transducer 22 tomove away from the electrodes 20 b of the first transducer 20. Thisresults in a lengthening of the “delay line”. The lengthening of thedelay line causes an decrease in the frequency at the output of theamplifier. The displacement of the elastic member is proportional to theweight of the applied load and the frequency or decrease in frequency atthe output of the amplifier can be calibrated to the distance moved bythe elastic member.

It will be appreciated that locating the transducers at the center ofthe elastic member compensates for any torque on the member which wouldexhibit itself at the free end of the member. This results in animproved accuracy as compared to the weighing instrument of the parentapplication. Depending on the application (e.g. maximum load to beweighed), the elastic member is made of aluminum or steel. The presentlypreferred elastic member exhibits a maximum displacement of 0.1 to 0.2mm at maximum load.

It has been recognized by the inventors that reflected waves may occuron the substrate of the transmitting transducer which interfere with SAWwave generation and result in non-linearity of measurements. Moreparticularly, when the wave 26 propagates along the substrate 20 a, itreaches the end 20 c of the substrate and a portion of the wave isreflected back 180° toward the electrodes 20 b. The reflected waveinterferes with the propagated wave 26. In fact, a portion of thereflected wave is again reflected off the other end 20 d of thesubstrate 20 a causing additional interference. Reflected waves can alsobe a problem in the receiving transducer. FIGS. 2-4 show severalanti-reflection structures according to the invention.

Turning now to FIG. 2, a transducer 120 according to the inventionincludes a piezoelectric substrate 120 a and a pair of electrodes 120 bfor generating a SAW wave 126. According to the invention, the end 120 cof the substrate 120 a is cut at an angle relative to the propagationpath of the SAW wave 126. Thus, when the wave 126 reaches the end 120 cof the substrate, any reflection of the wave is at an angle relative tothe line of propagation so that the reflected wave does not interferewith the propagated wave.

Turning now to FIG. 3, a transducer 220 according to the inventionincludes a piezoelectric substrate 220 a and a pair of electrodes 220 bfor generating a SAW wave 226. According to the invention, the end 220 cof the substrate 220 a is rounded (e.g., by sandblasting) relative tothe propagation path of the SAW wave 226. Thus, when the wave 226reaches the end 220 c of the substrate it is scattered rather thanreflected back.

Turning now to FIG. 4, a transducer 320 according to the inventionincludes a piezoelectric substrate 320 a and a pair of electrodes 320 bfor generating a SAW wave 326. According to the invention, a damper suchas a soft elastomeric 320 d is placed on the surface of the substrateadjacent the end 320 c. Thus, when the wave 326 reaches the damper 320d, it is absorbed by the damper rather than reflected back.

Of the different anti-reflection structure described above, the dampermaterial shown in FIG. 4 appears to be the presently preferredstructure. Accordingly, as shown in FIG. 5, a pair of transducers 320,322 are provided with damper material 320 d, 322 d adjacent ends 320 c,322 c opposite electrodes 320 b, 322 b on the respective substrates 320a, 322 a. The transducers 320, 322 are advantageously arranged in anoverlapping manner as shown in FIG. 5.

As mentioned above, in addition to problems associated with reflectedwaves, changes in temperature adversely affect the accuracy the weighingapparatus. It is known from the parent application that overall changesin ambient temperature may be compensated for by including a temperaturesensor in the weighing apparatus and using a look-up table forappropriate temperature corrections. However, in addition to overallchanges in ambient temperature, it has been discovered that temperaturegradients can occur across the substrates of the transducers. Moreparticularly, it has been discovered that the lithium niobate substrateshave a temperature effect of about 70 ppm per degree C. In a 20 Mhzsystem, this results in a change of 1.4 Khz per degree. In order toobtain the desired accuracy, the temperature difference between thetransducers should be less than 0.01 degrees C. At 20 Mhz, the fullscale output for one mode is about 200 Khz. Since the substrate shows atemperature effect of 1.4 Khz per degree C., this results in a 0.7%variation of the full scale per degree C. In order to maintain anaccuracy of within 0.007% of the full scale, therefore, the temperaturedifference should be less than 0.01 degree C.

Several aspects of the present invention combine to help overcome theeffects of temperature on the accuracy of the weighing apparatus. Someof these aspects also enhance the overall accuracy of the apparatusregardless of temperature effects.

FIGS. 6-8 show a pair of transducers 420, 422 according to the inventionwhich embody several aspects of the invention. Referring now to FIGS.6-8, the transducer system includes a transmitting transducer 420 and areceiving transducer 422. The transmitting transducer 420 includes twopiezoelectric substrates 420 a, 420 a′ which are mounted on one side athermal sink 421, and a thermal insulator 423 is mounted on the otherside of the thermal sink 421. The thermal sink 421 helps to maintain aconstant temperature across both substrates 420 a, 420 a′ and thethermal insulator 423 helps to prevent ambient temperature changes fromaffecting the temperature of the thermal sink 421 and thus thesubstrates 420 a, 420 a′. Each substrate 420 a, 420 a′ has a pair ofelectrodes 420 b, 420 b′ at one end thereof and a surface damper 420 d,420 d′ at the other end thereof. The substrates are arranged on thethermal sink so that their respective electrode pairs are close to eachother and adjacent to the center of the transducer as seen best in FIGS.6 and 7. The transmitting transducer 420, therefore generates two SAWwaves 426, 426′ which propagate in opposite directions from theapproximate center of the transducer 420. The dampers 420 d, 420 d′serve to inhibit reflections of the SAW waves 426, 426′.

The receiving transducer 422 includes a piezoelectric substrate 422 awhich is mounted on one side of a thermal sink 425, and a thermalinsulator 427 is mounted on the other side of the thermal sink 425. Thethermal sink 425 helps to maintain a constant temperature across thesubstrate 422 a and the thermal insulator 427 helps to prevent ambienttemperature changes from affecting the temperature of the thermal sink425 and thus the substrate 422 a. The substrate 422 a has two pairs ofelectrodes 422 b, 422 b′, each pair being located approximately atopposite rounded ends 422 c, 422 c′ of the substrate 422 a, and asurface damper 422 d is located approximately at the center of thesubstrate 422 a. The receiving transducer 422, therefore receives twoSAW waves 428, 428′ which are induced respectively by the transmittedSAW waves 426, 426′, and which propagate in opposite directions.

The electrode pairs 420 b and 422 b are coupled to one amplifier to formone delay line oscillator and the electrode pairs 420 b′ and 422 b′ arecoupled to a second amplifier to form a second delay line oscillator.From the foregoing, it will be appreciated that when the transducers420, 422 move relative to each other as described above with referenceto FIG. 1, one of the delay line oscillators will increase in frequencyand the other delay line oscillator will decrease in frequency by anequal amount. This “differential” transducer system provides severalbenefits. The effects of ambient temperature are automatically accountedfor and the resolution of the displacement measurement is doubled.

For example, if the frequency of a delay line oscillator is f₀ at apredefined temperature, the frequency after a change in ambienttemperature can be expressed as f=f₀(1+kΔt), where k is a constant andΔt is the change in ambient temperature. Similarly, if the frequency ofa delay line oscillator is f₀ when no load is placed on the weighingapparatus, the frequency after the transmitter and receiver have beendisplaced by a load can be expressed as f=f₀(1±gΔx), where g is aconstant and Δx is the displacement (positive or negative) due to aparticular load. The combined change in frequency Δf due to temperatureand displacement, therefore equals f₀(kΔt±gΔx).

Given the foregoing, it will be appreciated that the combined effects oftemperature and displacement on the frequency of the delay lineoscillators of FIGS. 6-8 can be expressed by the equations indicatedbelow at (1) and (2) where f1 is the frequency of the oscillator whichincludes electrodes 420 b, 422 b and f2 is the frequency of theoscillator which includes electrodes 420 b′, 422 b′.

 Δf1=f1₀kΔt+f1₀gΔx  (1)

Δf2=f2₀kΔt−f2₀gΔx  (2)

The quantities Δf1, Δf2, f1₀, and f2₀, will therefore be known duringany weight measurement and may be subjected to the cross productexpressed below at (3).

(Δf1×f2₀)−(Δf2×f1₀)  (3)

By substituting equations (1) and (2) in expression (3), the expressionlisted below at (4) is obtained.

f2₀f1₀kΔt+f2₀f1₀gΔx−f1₀f2₀kΔt+f1₀f2₀gΔx  (4)

By combining terms, it will be appreciated that the effects oftemperature will be canceled out of expression (4) and that expressions(3) and (4) can be expressed as the simplified equation listed below at(5).

(Δf1×f2₀)−(Δf2×f1₀)=2f1₀f2₀gΔx  (5)

It will therefore be appreciated that the differential transducer systemdescribed above, not only eliminates the affects of ambient temperaturechange from the weighing measurement, but also provides double theresolution scale of a single transducer system. Nevertheless, the systemdescribed with reference to FIGS. 6-8 will not automatically compensatefor temperature gradients across the substrates of the transducer andthat is why the transducer system is provided with the thermal sinksdescribed above. It will also be appreciated that the overall length(height) of the transducer system is substantially doubled as comparedwith the non-differential system. In order to reduce the overall size ofthe transducer system and to increase the sensitivity and resolution ofthe system, the present invention preferably uses a frequency ofapproximately 50 mhz as compared to the 20 mhz frequency of the parentapplication. At this frequency, in order to achieve the desired accuracy(0.5 grams per 10 kg), the temperature difference between the substratesmust be kept below 0.01 degrees C°.

FIGS. 9-11 show a schematic illustration of a differential transducersystem similar to the one described above, but with both transducerchannels operating on the same substrate and with one of the transducerchannels being split. Referring now to FIGS. 9-11, the transducer systemincludes a transmitting transducer 520 and a receiving transducer 522.The transmitting transducer 520 includes a piezoelectric substrate 520 awhich is mounted on one side a thermal sink 521, and a thermal insulator523 is mounted on the other side of the thermal sink 521. The thermalsink 521 helps to maintain a constant temperature across the substrate520 a and the thermal insulator 523 helps to prevent ambient temperaturechanges from affecting the temperature of the thermal sink 521 and thusthe substrate 520 a. A first transmitting electrode 520 b is located atone end of the substrate 520 a on a central axis thereof. A secondtransmitting electrode pair 520 c, 520 d is located at the other end ofthe substrate on opposite sides of the central axis. Each of theelectrodes 520 c, 520 d is approximately half the wavelength of theelectrode 520 b and the electrodes 520 c, 520 d are coupled in parallelto form the electrode pair. The transmitting transducer 520, thereforegenerates three SAW waves 526, 526′, and 526″. The first SAW wave 526propagates along a central channel on the substrate and in a firstdirection. The second and third SAW waves 526′ and 526″ propagate alongtwo side channels on the substrate an in a direction opposite to thefirst SAW wave.

The receiving transducer 522 includes a piezoelectric substrate 522 awhich is mounted on one side of a thermal sink 525, and a thermalinsulator 527 is mounted on the other side of the thermal sink 525. Thethermal sink 525 helps to maintain a constant temperature across thesubstrate 522 a and the thermal insulator 527 helps to prevent ambienttemperature changes from affecting the temperature of the thermal sink525 and thus the substrate 522 a. The substrate 522 a has a firstreceiving electrode 522 b and a second receiving electrode pair 522 c,522 d. The receiving electrode 522 b is at one end of the substrate on acentral axis thereof and the receiving electrodes 522 c, 522 d arelocated at the other end of the substrate on opposite sides of thecentral axis. The receiving electrodes 522 c, 522 d are half thewavelength of the electrode 522 b and are coupled in parallel to eachother. The receiving transducer 522, therefore receives three SAW waves528, 528′, and 528″ which are induced respectively by the transmittedSAW waves 526, 526′, 526″.

The arrangement shown in FIGS. 9-11 has several advantages. The SAW wavepropagation channels are closer together on the same substrate andtherefore the temperature gradient between them will be smaller. Theoverall size of the transducer system is smaller. In addition, thetransducers are axially symmetrical which enhances mechanicalperformance.

FIGS. 12-14 show a schematic illustration of a differential transducersystem similar to the one described above, but with additionalelectronic means for measuring the temperature of the substrates.Referring now to FIGS. 12-14, a transmitting transducer 620 includes afirst piezoelectric substrate 620 a and a second piezoelectric substrate620 a′, both of which are mounted on a base 623 which is preferablyconstructed as a sandwich of insulating and conductive materials asdescribed above with reference to FIG. 6. The first piezoelectricsubstrate 620 a is provided with a first pair of transmitting electrodes620 b at a lower central portion thereof and the second piezoelectricsubstrate 620 a′ is provided with a second pair of transmittingelectrodes 620 b′ at an upper central portion thereof. In addition,first piezoelectric substrate 620 a is provided with two pair ofelectrodes 630 a, 630 b which are spaced apart from each other andarranged off center from the first pair of transmitting electrodes 620b. The electrodes 630 a, 630 b are respectively a transmitter andreceiver which are used to measure the temperature of the substrate 620a as described below. Accordingly the substrate 620 a′ is also providedwith two pair of temperature measuring electrodes 630 a′, 630 b′.

A receiving transducer 622 includes a first piezoelectric substrate 622a and a second piezoelectric substrate 622 a′, both of which are mountedon a base 627 which is preferably constructed as a sandwich ofinsulating and conductive materials as described above with reference toFIG. 6. The first piezoelectric substrate 622 a is provided with a firstpair of receiving electrodes 622 b at an upper central portion thereofand the second piezoelectric substrate 622 a′ is provided with a secondpair of transmitting electrodes 622 b′ at a lower central portionthereof. In addition, the first piezoelectric substrate 622 a isprovided with two pair of temperature measuring electrodes 632 a, 632 bwhich are spaced apart from each other and arranged off center from thefirst pair of transmitting electrodes 522 b. The substrate 522 a′ isalso provided with two pair of temperature measuring electrodes 632 a′,632 b′.

The transducer arrangement shown in FIGS. 12-14 incorporates several ofthe features of the transducer arrangement described above withreference to FIGS. 6-8. The electrode pairs 620 b, 620 b′, 622 b, 622 b′are arranged to provide a differential displacement measurement systemas described above. In this regard, and with reference to FIG. 14, itwill be appreciated that the differential system utilizes two acousticchannels which are centrally located, one on the upper pair ofsubstrates and the other on the lower pair of substrates. In addition,it will be appreciated that the transmitting substrates are slightlylarger than the receiving substrates and therefore overlap the receivingsubstrates with the same advantages as described above. According to apresently preferred embodiment, the transmitting transducer 620 isapproximately 45 mm long and the space between the first and secondsubstrates is approximately 5 μm. The receiving transducer 622 isapproximately 10 mm shorter than the transmitting transducer and thespace between the first and second receiving substrates is approximately5 μm.

As mentioned above, each of the four piezoelectric substrates isprovided with a two pair of temperature measuring electrodes which arearranged as a fixed position delay line on each substrate. Each of thefour sets of temperature measuring electrodes is coupled to a respectiveamplifier and thereby forms a natural oscillator which preferablyoscillates at a frequency which is different from the frequency at whichthe displacement measuring oscillators oscillate. Since the temperaturemeasuring electrode sets are stationary on their respective substrates,the frequency of their respective oscillators will vary only due tochanges in temperature. With this provided arrangement, the temperatureof each of the four substrates can be determined and accounted for whenmaking displacement and weight measurements.

As seen in FIG. 14, each of the two measuring electrode sets operates ina separate acoustic channel. It is possible, however, to provide twomeasuring electrode sets which operate in almost the same channel. Thisminimizes the thermal gradient between the two channels. FIGS. 15-17illustrate one embodiment of such an arrangement.

Turning now to FIGS. 15-17, a transmitting transducer 720 includes apiezoelectric substrate 720 a with a pair of transmitting electrodes 720b located at a lower end thereof and a pair of receiving electrodes 730located at an upper end thereof. A receiving transducer 722 includes apiezoelectric substrate 722 a with a pair of receiving electrodes 722 bat an upper end thereof. It will be appreciated that the transmittingand receiving electrodes 720 b, 722 b are arranged with amplifier 750 toform a delay line for measuring displacement and weight as describedabove. In addition, the receiving electrodes 730 on the transmittingsubstrate form a stationary delay line with amplifier 760 and thetransmitting electrodes 720 b for measuring the temperature of thetransmitting substrate 720 a. The amplifiers 750 and 760 may be operatedsimultaneously if the delay lines have significantly differentfrequencies. Alternatively, the amplifiers 750, 760 may be switched onand off alternatingly. It will be appreciated that all of the electrodesoperate in the same acoustic channel. The transducer system shown inFIGS. 15-17 is a non-differential system wherein the temperature of thetransmitting substrate is assumed to be close to that of the receivingsubstrate. However, it is possible to apply the technology of thissystem to a differential system wherein separate measurements are madefor the two channels. Such a system is shown in FIGS. 18-20.

Turning now to FIGS. 18-20, a transmitting transducer 820 includes apiezoelectric substrate 820 a with a pair of transmitting electrodes 820b located at a lower end thereof and a pair of transceiving electrodes830 located at an upper end thereof. A receiving transducer 822 includesa piezoelectric substrate 822 a with a pair of receiving electrodes 822b at an upper end thereof and a pair of transceiving electrodes 832 at alower end thereof. It will be appreciated that the transmitting andreceiving electrodes 820 b, 822 b are arranged to form a delay line formeasuring displacement and weight as described above. In addition, thetransceiving electrodes 830 and 832 can be used to form a stationarydelay line with respective electrodes 820 b, 822 b or may be used witheach other to form a displacement measuring delay line which isdifferential to the delay line formed by electrodes 820 b, 822 b. Inoperation, the four electrode pairs may be multiplexed such that at onemoment, two differential displacement measuring delay lines areactivated and at another moment two stationary temperature measuringdelay lines are activated. In this manner, the temperature of eachsubstrate can be ascertained prior to or even during temperaturemeasurement. It will be appreciated that all of the electrodes operatein the same acoustic channel.

As mentioned above and in the parent application, the delay linesaccording to the invention may oscillate in more than one mode andwithin each mode, the gain will vary as the frequency changes. Accordingto the invention, the phase of the oscillator may be shifted ±180° inorder to increase gain (decrease loss). FIGS. 21-26 illustrate how themodes of oscillation change during weighing and how phase shifting canbe used to increase gain.

Referring now to FIG. 21, in the idle state, with no weight applied tothe scale, the delay line will oscillate at a frequency “f₀” which isshown in FIG. 21 as the point having the most gain (least loss). Theoptimal gain area of the graph of FIG. 21 is shown in the shaded areasurrounding f₀ and represents a range of ±100 Khz, for example. Thisarea is considered optimal not only because it is the area of leastloss, but because it is the area wherein the curve exhibits the least“non-linearity” and is least influenced by temperature. As described indetail in the parent application, the delay line may oscillate in any ofseveral modes and the modes are separated from each other by a phasedifference of 2π. In the example shown in FIG. 21, the frequency f₀ hasa lower mode f₀−2πω and a higher mode f₀+2πω, where ω is the intergerone, the phase difference between the modes representing approximately340 Khz in this example. As weight is added to the scale, the delay linewill oscillate at a higher frequency “f₀+n”. For example, after adding arelatively small weight, the frequency of oscillation will rise to f₀+70Khz which is shown in FIG. 22 as f₁.

Referring now to FIG. 22, it will be seen that the new frequency ofoscillation f₁ is still within the optimal gain area and the higher andlower modes of oscillation have greater loss than the mode at f₁. Itwill be appreciated, however, that with the addition of additionalweight, the frequency f will soon pass out of the optimal gain area. Forexample, an additional weight could shift the frequency an additional 50Khz to the position shown in FIG. 23.

Referring now to FIG. 23, an oscillation frequency f₂ which isapproximately 120 Khz higher than f₀, would move the frequency out ofthe optimal gain area. Nevertheless, as shown in FIG. 23, the higher andlower modes of oscillation would still show greater loss than thecentral mode at f₂. Those skilled in the art will therefore appreciatethat from this point onward, additional weight will raise the frequencyof oscillation though an increasingly high loss area until the lowermode achieves greater gain than the central mode which is shown in FIG.24.

Turning now to FIG. 24, an oscillation frequency f₃, which isapproximately 170 Khz higher than f₀ would cause a shift to the lowermode of oscillation f₃−2πω. It will be appreciated, however, that evenafter the mode shift, the frequency of oscillation must shift throughapproximately 70 Khz more before oscillation of the lower mode willoccur in the optimal gain area. According to the invention, therefore,it is possible to apply a phase shift of πω to the oscillator in orderto force early oscillation in the optimal gain area. For example, asshown in FIGS. 23 and 24 as soon as the oscillation frequency f₂exhibits loss which indicates it is no longer in the optimal gain area,a phase shift of −πω is applied. This causes the oscillator (delay line)to oscillate at f₂−πω which, as can be seen in FIG. 23, is within theoptimal gain area of the frequency response curve. This phase shift willmaintain the frequency of oscillation f₃−πω within the optimal area evenwhen the frequency rises to f₃ as shown in FIG. 24. Eventually, however,additional weight on the scale will raise the frequency of oscillationto a point where the phase shifted frequency is outside the optimal gainarea.

For example, as shown in FIG. 25, when the frequency of oscillation f₄is increased to approximately f₀+255 Khz, the phase shifted central modeof oscillation f₄−πω will exit the optimal gain area. At this point,according to the invention, the −πω phase shift is discontinued and theoscillator will oscillate in its lower mode f₄−2πω which is within theoptimal gain area. If additional weight is added to the scale, thefrequency of oscillation will continue to rise until the lower mode ofoscillation passes beyond the optimal gain area as shown in FIG. 26.

Referring now to FIG. 26, if the frequency f₅ is raised to approximatelyf₀+400 Hhz, the lower mode oscillation f₅−2πω will pass beyond theoptimal gain area. According to the invention, at this point, a −πωphase shift will be applied to the oscillator. This will cause the lowermode of oscillation to reside at f₅−3πω which is within the optimal gainarea.

FIG. 27 shows a simplified schematic diagram of a positive feedback loopwith phase shifting according to the invention. Referring now to FIG.27, a simplified delay loop according to the invention includes a firsttransducer 920, a second transducer 922, a first differential amplifier950, a second differential amplifier 952, a pair of matchingtransformers 954, 956, a frequency counter and amplifier controller 958,and an output processor and weight display 960. The first transducer 920includes a piezoelectric substrate 920 a and electrodes 920 b. Thesecond transducer 922 includes a piezoelectric substrate 922 a andelectrodes 922 b. The electrodes 920 b are coupled via the matchingtransformer 954 to the inputs of the differential amplifiers 950, 952 ina parallel manner. The electrodes 922 b are coupled to the outputs ofthe amplifiers 950, 952 via the matching transformer 956. As shown inFIG. 27, the polarity of the outputs of the amplifier 950 is opposite tothe polarity of the outputs of the amplifier 952. In addition, theenable input of each amplifier is coupled to the frequency counter andamplifier controller 958 which is also coupled to the outputs of theamplifiers. According to the invention, the amplifiers 950, 952 areturned on at one time by the frequency counter and amplifier controller958. It will be appreciated that the phase of the outputs of theamplifiers differs by 180° or π. Thus, in order to apply or remove aphase shift, one of the amplifiers is turned off and the other is turnedon. Those skilled in the art will appreciate that other circuits can beutilized to produce substantially the same type of phase shifting andthat the circuit of FIG. 27 is merely one example. According to theexample shown in FIG. 27, the frequency counter and amplifier controller958 monitors the output of the amplifier 950 and detects when thefrequency passes beyond the optimal gain area as described above, e.g.,increases by 100 Khz. When the frequency increases by a preselectedamount, the frequency counter and amplifier controller 958 turns offamplifier 950 and turns on amplifier 952. The frequency counter andamplifier controller 958 then monitors the output of amplifier 952.After the frequency increases by an additional preselected amount, e.g.100 Khz, the frequency counter and amplifier controller 958 turns offamplifier 952 and turns on amplifier 950. While the frequency counterand amplifier controller 958 is monitoring frequencies, the frequenciesare passed to the output processor and weight display 960 which analyzesthe frequency of oscillation, correlates the frequency with a particularweight according to the methods described in the parent application, anddisplays the weight.

Turning now to FIGS. 28-30, those skilled in the art will appreciatethat the transducers, e,g, transducer 1020 in FIG. 28 propagate SAWwaves in several directions. In a transducer having a so-called“uni-directional topology”, a primary SAW wave 1026 is propagated by theelectrodes 1020 b toward the edge 1020 c and is damped by theanti-reflection damper 1020 e as described above. Another, albeit loweramplitude SAW wave 1026 a is propagated in the opposite direction towardedge 1020 d. It is desirable that additional anti-reflection damping beprovided for this wave also. However, as can be seen from FIGS. 29 and30, there is no room between the transducers 1020 and 1022 to providedampers like 1020 e and 1022 e while still maintaining the close spacingbetween the transducers. FIG. 31 shows one solution to the problem.

As shown in FIG. 31, the transducer 1120 is provided with a thinanti-reflection damper 1120 f between the electrodes 1120 b and the edge1120 d. The damper 1120 f is made from a layer of MYLAR which isapproximately three microns thick. The MYLAR is glued to the substrate1120 a. One disadvantage of this solution is that the glue used to affixthe MYLAR is approximately seven microns thick. The resulting thicknessof approximately ten microns is too thick to maintain the optimal closespacing desired between two transducers. A different solution is shownin FIGS. 32 and 33.

The transducer 1220 shown in FIG. 32 is provided with a multistripcoupler 1220 g and an anti-reflection damper 1220 h which is similar insize to the damper 1220 e. The coupler 1220 g is made from an aluminizedpattern which is printed on the substrate 1220 a and which is designedto redirect SAW waves from the electrodes 1220 b toward the damper 1220h. According to a presently preferred embodiment, the coupler includesone hundred parallel lines spaced with a period of approximately 0.7times the wavelength of the SAW waves. Those skilled in the art willappreciate that types of couplers, using different patterns, can achievesimilar results.

As shown in FIG. 33, the arrangement of FIG. 32 allows the closeplacement of two transducers 1220 and 1222 with relatively thick dampers1220 e, 1220 h, 1222 e, 122 h while maintaining a close spacing betweenthe substrates 1220 a, 1222 a.

As mentioned above, in order to assure good long term stability, it isdesirable that the transducers be sealed. Turning now to FIG. 34, aweighing apparatus 110 is similar to the apparatus 10 shown in FIG. 1with similar reference numerals (increased by 100) referring to similarstructure. As shown in FIG. 34, transducers 620, 622 (having on boardtemperature sensors) are sealed by providing a sealing box 102 whichcovers the entire elastic member 114. A rolling diaphragm 104 permitsmovement of the elastic member 114 and the load platform 116 relative tothe box 102.

Another way to seal the transducers is shown in FIG. 35 whichillustrates a weighing apparatus 210 is similar to the apparatus 10shown in FIG. 1 with similar reference numerals (increased by 200)referring to similar structure. According to this embodiment, a flexiblesleeve 202 is placed over the transducers 620, 622 and sealed to theposts 217, 219. The sleeve may be made of a light weight LATEX orsimilar material. It will be appreciated that other methods of sealingthe transducers can yield similar results.

As mentioned above, the effects of temperature and long term degradationcan be further corrected by providing a seperate hermetically sealed SAWtemperature sensor in addition to the temperature sensors on board thetransducers in the elastic member. This is particularly useful if thetemperature sensors on board the transducers in the elastic member arenot hermetically sealed or if the seal (102, 202) is not perfectlyhermetic. According to this aspect of the invention, two weightcorrections can be made based on the temperature sensed by thehermetically sealed unit. As shown in equation 6, the effects oftemperature on the elastic member (114, 214) can be compensated for toyield a corrected weight Wc from a non-corrected weight Wn based on thetemperature To in ° C. of the hermetically sealed transducer at the timethe weight is measured and the temperature Tc in ° C. of thehermetically sealed transducer at the time the unit was calibrated. Theconstant 55×10⁻⁵ is based on a Youngs modulus as well as otherparameters for a particular aluminum elastic member. Other elasticmembers will require a different constant.

Wc=Wn+(Wn*(Tc−To)*(55×10⁻⁵)  (6)

It will be appreciated that equation 6 may be implemented using anyhighly accurate temperature sensor. As shown in equation 7, long termeffects (such as absorption of water vapor and other degredation effectsdue to incomplete sealing) on the weight measuring transducer can becompensated for to yield a corrected weight Wc based on the uncorrectedweight Wn, the reading Th (in Mhz) sensed by the hermetically sealedtransducer, and the reading Tn (in Mhz) sensed by the non-sealedtemperature sensor, where fc is the center frequency (in Mhz) for bothsensors. $\begin{matrix}{{Wc} = {{Wn} - \left( {{Wn}*\frac{{Th} - {Tn}}{fc}} \right)}} & (7)\end{matrix}$

There have been described and illustrated herein several embodiments ofan electronic weighing apparatus utilizing surface acoustic waves. Whileparticular embodiments of the invention have been described, it is notintended that the invention be limited thereto, as it is intended thatthe invention be as broad in scope as the art will allow and that thespecification be read likewise. Thus, while particular geometries of thebase, elastic member, and load platform have been disclosed, it will beappreciated that other geometries could be utilized. Also, whileparticular wavelengths have been disclosed, it will be recognized thatother wavelengths could be used with similar results obtained. Moreover,while particular configurations have been disclosed in reference to thelocation of transmitting and receiving electrodes, it will beappreciated that the respective locations of transmitters and receiverscould be reversed. Furthermore, while several different aspects of theinvention have been disclosed as solving various problems, it will beunderstood that the different aspects of the invention may be used incombination with each other in configurations other than those shown. Itwill therefore be appreciated by those skilled in the art that yet othermodifications could be made to the provided invention without deviatingfrom its spirit and scope as so claimed.

What is claimed is:
 1. An electronic weighing apparatus, comprising: a)a displaceable elastic member means for receiving a load and beingdisplaced by the load such that the displacement of said elastic membermeans is related to the weight of the load; b) a first piezoelectrictransducer having a first substrate and a first surface acoustic wave(SAW) transmitter, said first piezoelectric transducer being coupled tosaid elastic member; c) a second piezoelectric transducer having asecond substrate and a first SAW receiver, said second piezoelectrictransducer being mounted in close proximity to said first piezoelectrictransducer such that said displacement of said elastic member causes acorresponding displacement of one of said first and second piezoelectrictransducers relative to each other; d) a first amplifier having an inputand an output, said input of said first amplifier being coupled to saidfirst SAW receiver and said output of said first amplifier being coupledto said first SAW transmitter such that said first piezoelectrictransducer, said first amplifier, and said second piezoelectrictransducer form a first oscillator having a first output frequency; e)processor means coupled to said output of said first amplifier; and f)sealing means covering said first and second piezoelectric transducersfor sealing out moisture and other contaminants, wherein displacement ofsaid elastic member means causes a displacement of one of said first andsecond piezoelectric transducers relative to each other and therebychanges said first output frequency, and said first output frequency isused by said processor means to determine an indication of the weight ofthe load.
 2. An electronic weighing apparatus according to claim 1,wherein: said sealing means is an hermetic seal.
 3. An electronicweighing apparatus according to claim 1, wherein: said sealing means isa flexible sleeve.
 4. An electronic weighing apparatus, comprising: a) adisplaceable elastic member means for receiving a load and beingdisplaced by the load such that the displacement of said elastic membermeans is related to the weight of the load; b) a first piezoelectrictransducer having a first substrate and a first surface acoustic wave(SAW) transmitter, said first piezoelectric transducer being coupled tosaid elastic member; c) a second piezoelectric transducer having asecond substrate and a first SAW receiver, said second piezoelectrictransducer being mounted in close proximity to said first piezoelectrictransducer such that said displacement of said elastic member causes acorresponding displacement of one of said first and second piezoelectrictransducers relative to each other; d) a first amplifier having an inputand an output, said input of said first amplifier being coupled to saidfirst SAW receiver and said output of said first amplifier being coupledto said first SAW transmitter such that said first piezoelectrictransducer, said first amplifier, and said second piezoelectrictransducer form a first oscillator having a first output frequency; e)processor means coupled to said output of said first amplifier; and f)an hermetically sealed temperature sensor having an output coupled tosaid processor means, wherein displacement of said elastic member meanscauses a displacement of one of said first and second piezoelectrictransducers relative to each other and thereby changes said first outputfrequency, and said first output frequency is used by said processormeans to determine an indication of the weight of the load and saidprocessor means uses said output of said hermetically sealed temperaturesensor to compensate for the effects of temperature on said output ofsaid first amplifier.
 5. An electronic weighing apparatus, comprising:a) a displaceable elastic member means for receiving a load and beingdisplaced by the load such that the displacement of said elastic membermeans is related to the weight of the load; b) a first piezoelectrictransducer having a first substrate and a first surface acoustic wave(SAW) transmitter, said first piezoelectric transducer being coupled tosaid elastic member; c) a second piezoelectric transducer having asecond substrate and a first SAW receiver, said second piezoelectrictransducer being mounted in close proximity to said first piezoelectrictransducer such that said displacement of said elastic member causes acorresponding displacement of one of said first and second piezoelectrictransducers relative to each other; d) a first amplifier having an inputand an output, said input of said first amplifier being coupled to saidfirst SAW receiver and said output of said first amplifier being coupledto said first SAW transmitter such that said first piezoelectrictransducer, said first amplifier, and said second piezoelectrictransducer form a first oscillator having a first output frequency; ande) processor means coupled to said output of said first amplifier,wherein one of said first and second piezoelectric transducers isprovided with two anti-reflection structures to minimize reflection ofsurface acoustic waves, and displacement of said elastic member meanscauses a displacement of one of said first and second piezoelectrictransducers relative to each other and thereby changes said first outputfrequency, and said first output frequency is used by said processormeans to determine an indication of the weight of the load.
 6. Anelectronic weighing apparatus according to claim 5, wherein: one of saidtwo anti-reflection structures is a MYLAR film glued to said substrate.7. An electronic weighing apparatus according to claim 5, wherein: oneof said two anti-reflection structures is a surface damper on saidsubstrate with a multistrip coupler located between said surface damperand said SAW transmitter or receiver.